1. Field of the Invention
This invention relates to the addition of small amounts of a vanadium containing material to the petroleum based feedstocks used for partial oxidation reactions. The vanadium additions facilitate deslagging of the partial oxidation reactor.
2. Description of the Prior Art
Petroleum based feedstocks include impure petroleum coke and other hydrocarbonaceous materials, such as residual oils and byproducts from heavy crude oil. These feedstocks are commonly used for partial oxidation reactions that produce mixtures of hydrogen and carbon monoxide gases, commonly referred to as "synthesis gas" or simply "syngas." Syngas is used as a feedstock for making a host of useful organic compounds and can also be used as a clean fuel to generate power. The syngas feedstocks generally contain significant amounts of contaminants such as sulfur and various metals such as vanadium, nickel and iron.
The charge, including feedstock, free-oxygen-containing gas and any other materials, is delivered to the partial oxidation reactor. The partial oxidation reactor is also referred to as a "partial oxidation gasifier reactor" or simply a "reactor" or "gasifier," and these terms are used interchangeably throughout the specification.
Any effective means can be used to feed the feedstock into the reactor. Generally, the feedstock and gas are added through one or more inlets or openings in the reactor. Typically, the feedstock and gas are passed to a burner which is located in the reactor inlet. Any effective burner design can be used to assist the addition or interaction of feedstock and gas in the reactor, such as an annulus-type burner described in U.S. Pat. No. 2,928,460 to Eastman et al., U.S. Pat. No. 4,328,006 to Muenger et al. or U.S. Pat. No. 4,328,008 to Muenger et al.
Alternatively, the feedstock can be introduced into the upper end of the reactor through a port. Free-oxygen-containing gas is typically introduced at high velocity into the reactor through either the burner or a separate port which discharges the oxygen gas directly into the feedstock stream. By this arrangement the charge materials are intimately mixed within the reaction zone and the oxygen gas stream is prevented from directly impinging on and damaging the reactor walls.
Any effective reactor design can be used. Typically, a vertical, cylindrically shaped steel pressure vessel can be used. Illustrative reactors and related apparatus are disclosed in U.S. Pat. No. 2,809,104 to Strasser et al., U.S. Pat. No. 2,818,326 to Eastman et al., U.S. Pat. No. 3,544,291 to Schlinger et al., U.S. Pat. No. 4,637,823 to Dach, U.S. Pat. No. 4,653,677 to Peters et al., U.S. Pat. No. 4,872,886 to Henley et al., U.S. Pat. No. 4,456,546 to Van der Berg, U.S. Pat. No. 4,671,806 to Stil et al. , U.S. Pat. No. 4,760,667 to Eckstein et al., U.S. Pat. No. 4,146,370 to van Herwijner et al. , U.S. Pat. No. 4,823,741 to Davis et al., U.S. Pat. No. 4,889,540 Segerstrom et al., U.S. Pat. No. 4,959,080 to Sternling, and U.S. Pat. No. 4,979,964 to Sternling. The reaction zone preferably comprises a downflowing, free-flow, refractory-lined chamber with a centrally located inlet at the top and an axially aligned outlet in the bottom.
The refractory can be any effective material for a partial oxidation reactor. The refractory can be prefabricated and installed, such as fire brick material, or may be formed in the reactor, such as plastic ceramic. Typical refractory materials include at least one or more of the following: metal oxides, such as chromium oxide, magnesium oxide, ferrous oxide, aluminum oxide, calcium oxide, silica, zirconia, and titania; phosphorus compounds; and the like. The relative amount of refractory materials may be any effective proportion.
The partial oxidation reaction is conducted under any effective reaction conditions, sufficient to convert a desired amount of feedstock to syngas. Reaction temperatures typically range from about 900.degree. C. to about 2,000.degree. C., preferably from about 1,200.degree. C. to about 1,500.degree. C. Pressures typically range from about 1 to about 250, preferably from about 10 to about 200, atmospheres. The average residence time in the reaction zone generally ranges from about 0.5 to about 20, and normally from about 1 to about 10, seconds.
The partial oxidation reaction is preferably conducted under highly reducing conditions for syngas production. Generally, the concentration of oxygen in the reactor, calculated in terms of partial pressure, during partial oxidation is less than about 10.sup.-5, and typically from about 10.sup.-12 to about 10.sup.-8 atmospheres.
The partial oxidation of impure petroleum coke or other suitable petroleum based feedstock that has contaminant materials produces a slag byproduct that can collect and build up deposits on the inside surface of the reactor or at the lower throat of the reactor and the reactor outlet to the extent that blockage can occur and effective partial oxidation is prevented. Therefore, periodic shutdown of the partial oxidation reactor becomes necessary to remove slag, in an operation commonly referred to as "controlled oxidation" or "deslagging." Controlled oxidation conditions in the partial oxidation reactor are used to fluidize or melt the slag so that it can be removed by flowing out of the reactor, and thereby enable the reactor to be restored to partial oxidation operation.
Petroleum based feedstocks such as impure petroleum coke generally contain vanadium as a primary ash constituent along with various amounts of alumina, silica, and calcium. During the partial oxidation reaction to form syngas, the alumina, silica and calcium constituents of the petroleum coke feedstock tend to form a siliceous glass matrix that surrounds the vanadium, which exists primarily in the form of vanadium trioxide (V.sub.2 O.sub.3) crystals.
The ash particles formed as a byproduct of the syngas reaction will impinge and adhere to the inside surface walls of the reactor and, depending on the ash fusion temperature, accumulate in the form of slag, or flow out of the reactor.
Thus, the slag is essentially fused mineral matter, a by-product of the slag-depositing material in the petroleum based feedstock. Slag can also contain carbon in the form of char, soot, and the like.
The composition of the slag will vary depending on the type of slag-depositing material in the petroleum based feedstock, the reaction conditions and other factors influencing slag deposition. Typically, slag is composed of oxides and sulfides of slagging elements. For example, slag derived from impure petroleum coke or resid usually contains siliceous material, such as glass and crystalline structures such as wollastinite, gehlenite and anorthite; vanadium oxide, generally in the trivalent state, V.sub.2 O.sub.3 ; spinel having a composition represented by the formula AB.sub.2 O.sub.4 wherein A is iron and magnesium and B is aluminum, vanadium and chromium; sulfides of iron and/or nickel; and metallic iron and nickel.
Slag having a melting temperature below the reactor temperature can melt and flow out of the reactor as molten slag. Since V.sub.2 O.sub.3 has a high melting point of about 1970.degree. C. (3578.degree. F.), greater amounts of V.sub.2 O.sub.3 in the slag will cause the melting temperature of the slag to increase.
Slag which has higher melting temperature than the reactor temperature generally builds up solid deposits in the reactor, typically adhering to the surfaces of the refractory material lining the reactor. Slag deposits increase as the partial oxidation reaction proceeds. The rate that slag accumulates can vary widely depending on the concentration of slag-depositing metal in the feedstock, reaction conditions, use of washing agents, reactor configuration and size, or other factors influencing slag collection.
The amount of slag accumulation eventually reaches a level where slag removal from the reactor becomes desirable or necessary. Although slag removal can be conducted at any time, the partial oxidation reaction is usually continued for as long as possible to maximize syngas production.